There are several types of fluid displacement apparatus which utilize an orbiting piston or fluid displacement member. One type is a rotary machine as described in U.S. Pat. No. 1,906,142 to John Ekelof, which includes an annular eccentrically movable piston that acts within an annular cylinder having a radial traverse wall. One end wall of the cylinder is fixedly mounted and the other wall consists of a cover disk connected to the annular piston which is driven by a crank shaft. Another prior art fluid displacement apparatus of the orbiting piston type is a scroll-type apparatus as shown in U.S. Pat. No. 801,182 to Creux. Though the present invention is applicable to either type of fluid displacement apparatus (i.e., using either an annular piston or a scroll-type piston), the description will be made in connection with a scroll-type compressor.
U.S. Pat. No. 801,182 discloses a device that includes two scrolls, each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that the spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby define and seal off at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll-type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Generally, in a conventional scroll-type fluid displacement apparatus, one of the scrolls is fixed to a housing and the other scroll, which is an orbiting scroll, is supported on a crank pin of a drive shaft at a location eccentric of the drive shaft's axis to cause the orbital motion of the orbiting scroll. The scroll-type apparatus also includes a rotation-preventing device which prevents the rotation of the orbiting scroll to thereby maintain the two scrolls in a predetermined angular relationship during the operation of the apparatus.
Furthermore, since the orbiting scroll is supported on the crank pin in a cantilever manner, an axial slant of the orbiting scroll occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the scroll, but orbiting motion caused by the eccentric movement of the crank pin driven by the rotation of the drive shaft. Several problems result from the occurrence of this axial slant including improper sealing of the line contacts, vibration of the apparatus during operation and noise caused by physical striking of the spiral elements. One simple and direct solution to these problems is the use of a thrust-bearing device for carrying the axial loads. Thus, scroll-type fluid displacement apparatus are usually provided with a thrust-bearing device within the housing.
One recent attempt to improve the rotation-preventing/thrust-bearing devices in scroll-type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 (Hidden et al.) and 4,259,043 (Hidden et al.), in which the rotation-preventing/thrust-bearing devices are integral with one another. The rotation-preventing/thrust-bearing device described in these U.S. Patents (see FIG. 7 of U.S. Pat. No. 4,259,043) includes one set of indentations formed on the end surface of the circular end plate of the orbiting scroll and a second set of indentations formed on the end surface of a fixed plate attached to the housing. A plurality of balls or spheres are placed between the indentations of both surfaces. All the indentations have the same cross-sectional configuration, and the center of all indentations formed on both end surfaces are located about circles having the same radius. As a result, the machining and fabrication of these indentations to the required accurate dimensions is very difficult and intricate.
With reference to FIGS. 1, 2, and 3, one solution to the above disadvantage will be described. FIG. 1 is a vertical sectional view of a scroll-type compressor, and FIG. 2 is an exploded perspective view of a rotation-preventing/thrust-bearing device used in the compressor. Rotation-preventing/thrust-bearing device 23' surrounds a boss 223' of an orbiting scroll 22' and includes an orbital portion, fixed portion and bearings, such as a plurality of balls. The fixed portion includes (1) an annular fixed race 231' having one end surface fitted against the axial end surface of an annular projection 112' of a front end plate 11', and (2) a fixed ring 232' fitted against the other axial end surface of fixed race 231'. Fixed race 231' and ring 232' are attached to the axial end surface of annular projection 112' by pins 233'. The orbital portion also includes (1) an annular orbital race 234' having one end surface fitted against the axial end surface of a circular end plate 221', and (2) an orbital ring 235' fitted against the other axial end surface of orbital race 234' to extend outwardly therefrom and cover the other axial end surface of orbital race 234'. A small clearance is maintained between the facing end surfaces of fixed ring 232' and orbital ring 235'. Orbital race 234' and orbital ring 235' are attached to the end surface of circular end plate 221' by pins 236'.
Fixed ring 232' and orbital ring 235' each have a plurality of holes or pockets 232a' and 235a' in the axial direction, the number of holes or pockets in each ring 232', 235' being equal. Bearing elements, such as balls or spheres 237', are placed between facing generally aligned pairs of pockets 232a', 235a' of fixed and orbital rings 232', 235', with the rings 232', 235' facing one another at a predetermined clearance.
With reference to FIG. 3, the operation of the rotation-preventing/thrust-bearing device 23' will be described. In FIG. 3, the center of orbital ring 235' is placed at the right side and the direction of rotation of the drive shaft is clockwise, as indicated by arrow A. When orbiting scroll 23' is driven by the rotation of the drive shaft, the center of orbiting ring 235' orbits about a circle of radius Ror (together with orbiting scroll 22'). However, a rotating force (i.e., moment), which is caused by the offset of the acting point of the reaction force of compression and the acting point of drive force, acts on orbiting scroll 22'. This reaction force tends to rotate orbiting scroll 22' in a clockwise direction about the center of orbital ring 235'. But as shown in FIG. 3, eighteen balls 237' are placed between the corresponding pockets 232a' and 235a' of rings 232' and 235'. In the position shown in FIG. 3, the interaction between the nine balls 237' at the top of the rotation-preventing/thrust-bearing device 23' and the edges of the pockets 232a', 235a' prevents the rotation of orbiting scroll 22'. The magnitude of the rotation-preventing forces are shown as Fc.sub.1 -Fc.sub.5 in FIG. 3.
In the construction, as described above, the rotation-preventing/thrust-bearing device 23' is made up of a pair of races and a pair of rings, with each race and ring formed separately. Therefore, the parts of the rotation-preventing/thrust-bearing device are easy to construct and the most suitable material for each part can be selected. However, each ring is attached by pins. The rotation-preventing force of the ring is thus transmitted to the attachment pins. Since the location at which the rotation-preventing force of the rings act on the respective attachment pins is spaced from the location at which the pins are attached to the orbiting scroll or housing, a moment is generated which acts on the pins. Therefore, stress is placed on the attachment pins and this stress is increased by impact load which occurs when the compressor is driven at high speed. Also, since the attachment pins receive the radial component and tangential component of rotation preventing force, precession of the pins is caused. As a result, the attachment pins tend to move toward an outer direction and come out the holes in which they are located.